1. Field of the Invention
The present invention relates to CMOS integrated circuitry for elimination of noise from signals received from a SCSI (Small Computer Systems Interface) Bus. More particularly, the present invention relates to such noise elimination circuitry for use on an input buffer of a SCSI bus controller integrated with other components onto a single integrated circuit chip.
2. Description of the Related Art
SCSI is a standard that allows users to easily add up to seven peripheral devices on a personal computer such as CD-ROM and high capacity disk drives. A SCSI adapter board in a personal computer provides a SCSI bus controller which provides digital address, data, and control signals to a SCSI bus on which peripheral devices reside.
FIG. 1 illustrates the configuration of a SCSI bus with connecting devices. As shown, the SCSI bus 100 supports up to seven devices 101-107, with a potential one meter length of cable between the devices. Terminators 108 and 109 are provided at the ends of the SCSI bus cable. Device 1, labeled 101 is the host computer which includes an output buffer 120 and input buffer 130 connected to the SCSI bus line. The signal from the input buffer 130 is fed through a filter 140 to SCSI controller 150. Because of noise on SCSI bus signals, particularly switching noise and ground bounce created by output buffers such as 120, the input buffers can generate erroneous signals. Filters, such as filter 140 have, thus, been used between the input buffer and internal components of the device connected to the SCSI bus such as controller 150. Such a filter is disclosed in U.S. Pat. No. 5,113,098 entitled "Glitch Remover Circuit For Transmission Links", to Teymouri incorporated herein by reference.
More recent devices which operate on a SCSI bus can now generate significantly more noise. See, for example, U.S. patent application Ser. No. 08/186,050 entitled "Integrated SCSI and Ethernet Controller", on a PCI Bus by Chih-Siung Wu, filed Jan. 24, 1994. By integrating a SCSI controller with an Ethernet Controller the transistor count of the device which connects to the SCSI bus increases from nearly 20,000 to over 100,000. By increasing the transistor count in this manner, particularly as integrated circuit processes move to a sub-micron levels, noise on the SCSI bus is increased to a level greater than current filtering can effectively eliminate.
The larger devices on the SCSI bus introduce larger loading and generate additional ground bounce and current switching noise. Additional noise is also created by reflections of the noise created by the larger devices on the SCSI bus due to the long SCSI bus cable lengths. With such additional noise not eliminated by current filtering, input buffers receiving signals from the SCSI bus may once again generate erroneous signals.
Circuitry which responds to such added switching, ground bounce, and reflection noise on a SCSI bus is therefore desired. Such circuitry is particularly desired to protect a SCSI controller logic against false input signal sensing without substantially complicating the SCSI controller design.